1. Field of the Invention
The present invention relates to an optical transmitter and more particularly to an optical transmitter a wavelength multiplexing optical communication system and an optical network using a plurality of pieces of signal light.
2. Related Art
Wavelength multiplexing optical communication, which transmits an optical signal with a plurality of wavelengths along the same optical transmission line, is an important technology for achieving higher performance and lower cost of communication technology and optical information processing technology. A first major object from this point of view is to control the wavelength of a light source for a plurality of channels, and a second major object is to provide an optical modulation means for achieving rapid optical modulation.
Controlling the wavelength of a light source for a plurality of channels in a wavelength multiplexing optical communication system is one of the most important techniques. The wavelength c)r frequency of each channel used is now precisely determined by an international standardization (ITU) at an interval of 50 GHz to 100 GHz (about 0.4 nm to 0.8 nm)
Light sources considered for wavelength multiplexing optical communication with wavelengths controlled in the above manner include a wavelength variable type light source wherein a plurality of channels are covered by using a semiconductor laser device with a wavelength-variable function, and a wavelength selection type light source wherein a plurality of semiconductor laser devices with different wavelengths are used. Realization of these techniques requires realization of a wavelength variable type semiconductor laser that makes it possible to sweep wavelengths efficiently, readily, and continuously, as well as a wavelength control technique for controlling a wavelength on the ITU grid with high reproducibility, or a wavelength stabilization technique for fixing a wavelength on the ITU grid over a long period of time.
On the other hand, the typical transmission speed in a long-distance main line system is currently 2.5 Gb/s to 10 Gb/s. Because of this high-speed transmission, the optical modulation system needs to be an external modulation system, in which optical modulation is performed outside of the light-emitting device, rather than a system in which modulation is performed directly by the light-emitting device. Thus the optical modulation means for achieving rapid optical modulation requires the form of a modulator-integrated light source, in which an optical modulator is monolithically integrated with a laser device. A wavelength selection type light source which, from this point of view, monolithically integrates a plurality of semiconductor laser devices having different wavelengths with an optical multiplexer, an electric field absorption type optical modulator, and the like has been reported. In this example, however, there is a problem in that the characteristics of the optical modulator are changed when the wavelength of the laser light led to the optical modulator is varied.
Incidentally, this type of wavelength multiplexing communication light source is reported in the Institute of Electronics, Information and Communication Engineers general convention SC-3-5, 1999.
As for the above-mentioned first object of controlling the wavelength of a light source for a plurality of channels, one of the major problems is how to set the wavelength of a semiconductor laser device to be used as the light source at a standard value. Currently the standard value is practically provided by the ITU grid. In order to solve the problem, the following two measures are provided for a current system. One is to monitor the wavelength of each channel and provide a wavelength stabilizing means for the light source, which feeds back the correction based on the result of monitoring to the drive of the light source. The other is to provide a spare light source device for each channel in case an individual channel device of such a complex light source should fail.
With the background of these techniques, each of a plurality of DFB (Distributed Feedback) laser devices that corresponds to each channel in the light source used for a wavelength multiplexing optical communication system needs to be built so as to fit into a specified narrow wavelength range. Therefore, there is a major problem in the yields in terms of device manufacturing.
This problem of wavelength control is a major obstacle to the achievement of smaller size and lower cost of an optical transmitter, and will also be a major object to further reduction in channel intervals and the increase in the number of channels in the future.
Next the above-mentioned second object of providing a means for rapid modulation will be dealt with. The characteristics of a current optical modulation means change mainly for the following reason. Optical modulators formed by semiconductor materials include electric field absorption type modulators, optical interference type modulators, and phase modulators. The optical modulation characteristics of these optical modulators formed by semiconductor materials, however, depend heavily on the difference between the band gap energy of the semiconductor material and the energy of the signal light (amount of detuning). Therefore, as the wavelength of the signal light is changed, the amount of detuning of the optical modulator is greatly changed.
In addition, chirping in particular becomes a problem in the case of long-distance transmission. It is desirable from this point of view that the chirping parameter of an optical modulator can be varied according to the transmission specifications. In the case of an electric field absorption type optical modulator, the chirping parameter can be changed by offset voltage. In this case, however, the laser light output is reduced especially when the offset voltage is increased. This is caused by optical absorption in the optical wave guide of the optical modulator.
A first object of the present invention is to provide an optical transmitter that will solve problems such as the wavelength control of a light source, the provision of an optical modulation means for achieving rapid optical modulation, and the control of the chirping characteristics of an optical modulator.
A second object of the present invention is to provide an optical transmitter suitable for realizing a light source for wavelength multiplexing communication with high reproducibility and provide a wavelength multiplexing transmission apparatus using the optical transmitter. The second object of the present invention will be achieved by using a simple method. The method will also enable minute tuning in optical communication, and enable especially the tuning of a wavelength standard channel that conforms to the ITU grid.
A third object of the present invention is to provide an optical transmitter suitable for long-distance transmission and a simple means for realizing the transmitter. More specifically, it controls the chirping parameter of a light source including an optical modulator.
The present invention will be useful especially for a complex light source using a semiconductor laser device with an external modulator monolithically integrated.
First the basic technical concept of the present invention will be described. The basic configuration of the present invention is a combination of a wavelength variable type light source or an integrated type multiple wavelength light source and an optical modulator. The optical combination form of the light source and the optical modulator may be either a form in which they are separated from each other hybrid integration) or a form that monolithically integrates each other (monolithic integration). Incidentally, the configuration itself in which a light source that emits light with a plurality of wavelengths and an optical modulator are integrated on a single substrate is already known. Specifically, in such an apparatus, part of the output light from a wavelength variable type light source or an integrated type multiple wavelength light source is led to an optical modulator to generate an optical signal by the on and off of the optical modulator.
Another object of the present invention is to provide a means for locally controlling the temperature of the optical modulator in the proximity of such an optical modulator. The temperature controlling means controls the temperature of an active wave guide layer in the optical modulator to a desired value. The basic form of the present invention will also help solve problems such as the wavelength control of a light source, the provision of an optical modulation means for achieving rapid optical modulation, and the control of the chirping characteristics of an optical modulator.
In a first specific example of operation according to the present invention, the band gap wavelength of an optical modulator is varied according to and to the same degree as variations in the wavelength of an optical signal introduced into the optical modulator, for example. To illustrate a more specific embodiment of the present invention, the temperature control means at least has a means to detect the wavelength of the light incident on the optical modulator, and a means to control the operating temperature of the optical modulator based on the detected wavelength. In practical use, the temperature control means further has a means to control the operating current of the semiconductor laser device based on the detected wavelength. The means to control the operating temperature of the optical modulator varies the band gap wavelength of the optical modulator according to and to the same degree as variations in the wavelength of an optical signal introduced into the optical modulator. For example, when the wavelength of the optical signal introduced into the optical modulator is increased, the band gap wavelength of the optical modulator is increased to the same degree. The means to control the operating current of the semiconductor laser device controls minute variations in oscillation power based on the wavelength variations.
The above means make it possible to provide rapid optical modulation that enables the wavelength control of a light source. According to the present invention, variations in the optically modulated waveform can be controlled to a low level even when the wavelength of signal light is changed. Furthermore, according to the present invention, the chirping parameter is held to a specified value, and variations in the chirping parameter can be controlled to a low level. That is, the modulation characteristics of light are controlled while variations in the chirping parameter required are controlled to a low level. Thus the present invention makes it possible to provide high-quality fiber signal transmission.
Next, in the other form of operation according to the third object of the present invention, attention is directed to the chirping parameter of the optical modulator as an object to be controlled. In this case, the chirping parameter can be controlled whether the light emitter is of the wavelength variable type or of the wavelength fixed type. Specifically, in the other form of operation according to the present invention, the wavelength of laser light is practically constant, and a temperature control means is provided in the proximity of the optical modulator, the temperature control means making it possible to change the chirping parameter of the optical modulator by controlling the temperature of the optical modulator.
The basic concepts of the present invention has been described. Next essential aspects of the present invention will be described.
According to a first aspect of the present invention, there is provided an optical transmitter comprising: a semiconductor laser device; an optical modulator region optically connected to the semiconductor laser device and having the function of modulating the output light from the semiconductor laser device; and a means for changing the operating temperature of the optical modulator region without changing the oscillation wavelength of the semiconductor laser device.
Or there is provided an optical transmitter comprising: a semiconductor laser device; an optical modulator region optically connected to the semiconductor laser device and having the function of modulating the output light from the semiconductor laser device; and a means for changing the operating temperature of the optical modulator region while maintaining a desired oscillation wavelength of the semiconductor laser device.
According to a second aspect of the present invention, there is provided an optical transmitter comprising: a semiconductor laser device; an optical modulator region optically connected to the semiconductor laser device and having the function of modulating the output light from the semiconductor laser device; and a means for externally changing the oscillation wavelength of the semiconductor laser device, and for changing the operating temperature of the optical modulator region without changing the oscillation wavelength of the semiconductor laser device.
Or there is provided an optical transmitter comprising: a semiconductor laser device; an optical modulator region optically connected to the semiconductor laser device and having the function of modulating the output light from the semiconductor laser device; and a means for externally changing the oscillation wavelength of the semiconductor laser device, and for changing the operating temperature of the optical modulator region while maintaining a desired oscillation wavelength of the semiconductor laser device.
According to a third aspect of the present invention, there is provided an optical transmitter comprising: a semiconductor laser device; an optical modulator region optically connected to the semiconductor laser device and having the function of modulating the output light from the semiconductor laser device; and a means for controlling the modulation characteristics of the optical modulator as the wavelength of the light incident on the optical modulator region is changed.
According to a fourth aspect of the present invention, there is provided an optical transmitter comprising: a semiconductor laser device; an optical modulator region optically connected to the semiconductor laser device and having the function of modulating the output light from the semiconductor laser device; and a means for externally changing the oscillation wavelength of the semiconductor laser device, and for controlling the modulation characteristics of the optical modulator as the wavelength of the light incident on the optical modulator region is changed.
According to a fifth aspect of the present invention, there are provided the optical transmitters wherein a heating element serves as the means for controlling the modulation characteristics of the optical modulator as the wavelength of the light incident on the optical modulator region is changed. A typical example of the heating element is a heater.
The above aspects of the present invention are applicable in both cases where the semiconductor laser device is the one with a single fixed wavelength and where it is a wavelength multiplexing semiconductor laser device. In these cases, it is important to locally change the temperature of the optical modulator. On the other hand, it is also important that the oscillation wavelength of the semiconductor laser device be practically set at a desired value. Thus it is required that the temperature of the optical modulator region be controlled without changing the oscillation wavelength from the semiconductor laser device. In other words, in the above aspects of the present invention the temperature of the optical modulator region must be controlled without changing the chirping parameter (xcex1p).
Other aspects of the present invention described below are provided for the third object of the present invention.
In the above-described aspects of the present invention provided for the second object, the temperature of the optical modulator region is controlled in such a way that the chirping parameter (xcex1p) is not changed. However, in order to achieve the third object of the present invention, the chirping parameter is adjusted to a desired value. This renders these aspects of the present invention useful for multiple wavelength long-distance transmission.
Specifically, according to another aspect of the present invention, there is provided an optical transmitter comprising: a semiconductor laser device; an optical modulator region optically connected to the semiconductor laser device and having the function of modulating the output light from the semiconductor laser device; and a means for changing the operating temperature of the the optical modulator region without practically changing the oscillation wavelength of the semiconductor laser device, and adjusting the chirping parameter to a desired value. Of course in this case, the optical modulator has specified modulation characteristics.
According to another aspect of the present invention, there is provided an optical transmitter comprising: a semiconductor laser device; an optical modulator region optically connected to the semiconductor laser device and having the function of modulating the output light from the semiconductor laser device; and a means for externally changing the oscillation wavelength of the semiconductor laser device, and for changing the operating temperature of the optical modulator region without changing the oscillation wavelength of the semiconductor laser device and adjusting the chirping parameter to a desired value.
According to another aspect of the present invention, there is provided an optical transmitter comprising: a semiconductor laser device; at optical modulator region optically connected to the semiconductor laser device and having the function of modulating the output light from the semiconductor laser device; and a means for externally changing the oscillation wavelength of the semiconductor laser device, and for changing the operating temperature of the optical modulator region while maintaining the oscillation wavelength of the semiconductor laser device within xc2x11 nm.
According to another aspect of the present invention, there is provided an optical transmitter comprising a means for adjusting the chirping parameter of the optical modulator to a desired value by changing the operating temperature of the optical modulator.
The optical modulator, which is useful for inventions pertinent to the above-described optical transmitter of the present invention, controls a signal in a wavelength band in the proximity of the band gap in the light-emitting active region of the semiconductor light-emitting device whose light is made incident on the optical modulator. Optical modulators comprising such semiconductor materials include electric field absorption type modulators, optical interference type modulators, and phase modulators. The basic configuration of these optical modulators can be sufficiently realized by using normal materials, except for temperature control means.
In addition, the specific configuration of the above-mentioned semiconductor laser device can be selected as desired, depending on the purpose. For example, the configurations of the semiconductor laser device include the DFB (Distributed Feedback) type and the DBR (Distributed Bragg Reflection) type, each of which has a normal diffraction grating, or the one with a diffraction grating using an array wave guide. The active layer region can be configured as commonly known structures, depending on the purpose. The structures of the active layer region generally include a quantum-well structure, a multi-quantum well structure, a strained quantum-well structure, and a strain compensation type quantum-well structure. Furthermore, the emitted light can be used regardless of whether it is non-modulated light or modulated light.
In addition, it is practical to mount the entire module of an optical transmitter according to the present invention on a temperature control means or, for example, a Peltier element. Temperature stabilization by means of a Peltier element or the like is a common technique employed in semiconductor laser devices. However, the present invention is intended to locally control the temperature of the semiconductor laser device and the optical modulator region, independently of the temperature control by a temperature stabilization means provided for the entire module.
An optical transmitter or an optical transmission system according to the present invention will be described below.
The optical transmitters described above are useful in realizing a wavelength multiplexing optical communication apparatus which transmits information by propagating a light wave signal with at least two different wavelengths or more on the same optical transmission line. Also, the optical transmitters for the above wavelength multiplexing optical communication apparatus are useful especially when the interval between adjacent channels is 50 GHz to 100 GHz. Thus the optical transmitters according to the present invention are useful in satisfying the requirements for the wavelength or frequency of a channel specified by ITU.
Moreover, in the above optical communication apparatus, the above optical transmitters are extremely useful when a spare light source that can vary its oscillation wavelength is to be provided as an alternative light source in case a failure should occur in a channel light source. More specifically, if a failure occurs in the light source of a channel, the spare light source is operated. Then the system can be readily restored by using the wavelength variable function of the spare light source to perfectly match the wavelength of the spare light source to that of the channel whose light source has failed.
Furthermore, utilization of the present invention will realize an optical transmission apparatus which comprises any one of the optical transmitters described above, wherein information is transmitted by propagating a light wave signal with at least two different wavelengths or more on the same optical transmission line, the oscillation wavelength of the light source for each channel that the optical transmitter can be made variable, and a desired channel wavelength can be changed as necessary by an external signal.